This invention is generally directed to liquid developer compositions and, more specifically, to liquid developers with mixtures of nonpolar fluids, especially hydrocarbon mixtures, like ISOPAR.RTM. and mineral oil, wherein the volatiles, especially low molecular weight volatiles, are removed. More specifically, the present invention is directed to liquid developers containing hydrocarbons wherein undesirable volatiles, such as those of especially high vapor pressure, have been removed by, for example, heating, distillation and the like, resulting in a hydrocarbon mixture with retention times in a gas chromatographic test of, for example, not less than about 13 minutes. In embodiments, the nonpolar fluids are also separated or fractionated to remove the high molecular weight components of the mixture. The liquid inks of the present invention possess excellent viscosity, for example ISOPAR M.TM. provides a viscosity substantially unchanged from the ISOPAR M.TM. product as supplied by Exxon Corporation, and superior vapor pressures of 80 ppm at 20.degree. C. In embodiments, the liquid inks of the present invention contain a carrier liquid, colorant, resin, charge control agent, a charge director, and optionally a charge adjuvant. Charge directors such as those comprised of diblock or triblock copolymers of the formula A-B, BAB or A-B-A wherein the polar A block is an ammonium containing segment and B is a nonpolar block segment which, for example, provides for charge director solubility in the liquid ink fluid like ISOPAR.TM., and wherein the A blocks have a number average molecular weight range of from about 200 to about 120,000; the B blocks have a number average molecular weight range of from about 2,000 to 190,000; the ratio of M.sub.w to M.sub.n is 1 to 5 for the copolymers; and the total number average molecular weight of the copolymer is, for example, from about 4,000 to about 300,000, and preferably about 100,000. The developers of the present invention can be selected for a number of known imaging and printing systems, including high speed, for example over 70 copies per minute, printing, such as xerographic processes, wherein latent images are rendered visible with the liquid developers illustrated herein.
The image quality, solid area coverage and resolution for developed images usually require sufficient toner particle electrophoretic mobility. The mobility for effective image development is primarily dependent on the imaging system selected. The electrophoretic mobility is primarily directly proportional to the charge on the toner particles and inversely proportional to the viscosity of the liquid developer fluid. A 10 to 30 percent change in fluid viscosity caused, for instance, by a 5.degree. C. to 15.degree. C. decrease in temperature could result in a decrease in image quality, poor image development and background development, for example, because of a 5 percent to 23 percent decrease in electrophoretic mobility. Insufficient particle charge can also result in poor transfer of the toner to paper or other final substrates. Poor or unacceptable transfer can result in, for example, poor image developer solid area coverage when insufficient toner is transferred to the final substrate and can also cause image defects such as smears and hollowed fine features.
High vapor concentration of the nonpolar carrier is also disadvantageous. Indoor air quality concerns dictate that printing devices employing nonpolar liquid carrier liquids should not emit significant amounts of the vapor of the carrier fluid into the surrounding environment. High volatility carrier fluids such as ISOPAR H.RTM. require expensive removal of the volatile carrier liquid vapors from the air stream of the machine. A noble metal catalyst may be used to oxidize the hydrocarbon carrier to CO.sub.2 and water. Such a remediation device adds substantially to the cost of the printing device. Alternatively, the vapors can be recovered. Chilling of the air stream from the printing device condenses both the carrier liquid and water from the paper that the images are being fused to and from the ambient air. Both of these actions which are inextricably linked without separation of the water vapor from the air stream by means of, for instance, a chemical moisture filter, are very costly. The energy required for these condensation processes acids to the total cost of printing, and there is an added cost for disposing of the commingled water-carrier liquid waste stream that is produced. Alternatively the vapors of the carrier liquid can be absorbed on a bed of activated charcoal or other suitable media form which the carrier liquid can either be recovered by appropriate desorbtion procedure or disposed of when the bed becomes saturated. Either of these options add to the cost of the printing process and so are not desirable.
A desirable carrier liquid mixture will be substantially free of volatile components rendering these air quality remediation steps unnecessary.
Examples of acceptable conductivity and mobility ranges for the liquid developers of the present invention are as illustrated herein. These are in embodiments dependent upon the speed at which the printing of developed images is accomplished, and upon the mechanical and electrostatic variables (development potential and developer subsystem design which are to be used.
Conductivities, measured at ambient temperature (21.degree. C. to 23.degree. C.), for developers containing one percent toner solids are considered to be in the high range at 14 to 100 pmhos/centimeters. Medium conductivities are from about 6 to about 13 pmhos/centimeters, and low conductivities are from 0.1 to about 6 pmhos/centimeters. As conductivities increase into the undesirable high range, excess ions can compete with toner particles of the same charge for development of the latent image causing low developed mass resulting in low print density images. Also, with a low to medium conductivity of less than 14 pmhos/centimeter, the liquid toner or developer of this invention can possess a mobility of between about -1 to 1.99.times.10.sup.-10 m.sup.2 /Vs, and preferably -2.00 to 2.49.times.10.sup.-10 m.sup.2 /Vs, and more preferably -2.50 to 5.times.10.sup.-10 m.sup.2 /Vs.
The viscosity of the developer is also important. For example, the speed at which high quality copies, or prints can be obtained in a given device with given electrostatics is determined to a great extent by the viscosity of the carrier liquid. In a specific printing device, with specified electrostatic conditions and an ink with a given zeta potential, the maximum speed at which high quality print can be obtained is influenced by the carrier liquid viscosity. As the viscosity of that career liquid increases, the maximum speed at which high quality printing can be accomplished decreases. For a given nonpolar carrier liquid mixture, it is important to minimize the viscosity to, for example, enable an excellent printing speed range.
The above and other advantages are achievable with the liquid inks of the present invention.
A latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. The aforementioned dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be generated by providing a photoconductive layer with a uniform electrostatic charge, and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are also known for forming latent electrostatic images such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. After the latent image has been formed, it is developed by colored toner particles dispersed in a nonpolar liquid. The image may then be transferred to an intermediate member for transfer to a receiver sheet, or it can be directly transferred to a receiver sheet.
Liquid developers can comprise a thermoplastic resin and a dispersant nonpolar liquid. Generally, a suitable colorant, such as a dye or pigment, is also present in the developer. The colored toner particles are dispersed in a nonpolar liquid which generally has a high volume resistivity in excess of 10.sup.9 ohm-centimeters, a low dielectric constant, for example below 3.0, and a high vapor pressure. Generally, the toner particles are less than 10 microns (pro) average by area size as measured using the Horiba Capa 500 or 700 particle sizer.
Since the formation of images depends, for example, on the difference of charge between the toner particles in the liquid developer and the latent electrostatic image to be developed, it has been found desirable to add a charge control agent, charge director compound and charge adjuvants which increase the magnitude of the charge on the developer particle. Charge adjuvants such as polyhydroxy compounds, amino alcohols, polybutylene succinimide compounds, aromatic hydrocarbons, metallic soaps, and the like may be added to the liquid developer comprising the thermoplastic resin, the charge control agent, the charge director, the nonpolar liquid and the colorant. Other additives, such as those that modify slip or gloss, may optionally be added. Specifically, titania, silicas and waxes are common additives, but many are known in the art.
U.S. Pat. No. 5,019,477, the disclosure of which is totally incorporated herein by reference, illustrates a liquid electrostatic developer comprising a nonpolar liquid, thermoplastic resin particles, and a charge director. The ionic or zwitterionic charge directors disclosed may include both negative charge directors such as lecithin, oil-soluble petroleum sulfonate and alkyl succinimide, and positive charge directors such as cobalt and iron naphthanates. The thermoplastic resin particles can comprise a mixture of (1) a polyethylene homopolymer or a copolymer of (i) polyethylene and (ii) acrylic acid, methacrylic acid or alkyl esters thereof, wherein (ii) comprises 0.1 to 20 weight percent of the copolymer; and (2) a random copolymer of (iii) selected from the group consisting of vinyl toluene and styrene, and (iv) selected from the group consisting of butadiene and acrylate.
U.S. Pat. No. 5,030,535 discloses a liquid developer composition comprising a liquid vehicle, a charge control additive and toner particles. The toner particles of resin and optional charge adjuvant may contain pigment particles, wherein the resin can be selected from the group consisting of polyolefins, halogenated polyolefins and mixtures thereof, and in embodiments thermoplastics generally. The liquid developers are prepared by first dissolving the polymer resin in a liquid vehicle by heating at temperatures of from about 80.degree. C. to about 120.degree. C., adding pigment to the hot polymer solution and attriting the mixture, and then cooling the mixture so that the polymer becomes insoluble in the liquid vehicle, thus forming an insoluble resin layer around the pigment particles.
U.S. Pat. No. 5,026,621 discloses a toner for electrophotography which comprises as main components a coloring component and a binder resin which is a block copolymer comprising a functional segment (A) of at least one of a fluoroalkylacryl ester block unit or a fluoroalkyl methacryl ester block unit, and a compatible segment (B) of a fluorine-free vinyl or olefin monomer block unit. The functional segment of the block copolymer is oriented to the surface, and the compatible segment thereof is oriented to be compatible with other resins and a coloring agent contained in the toner so that the toner is provided with both liquid repelling and solvent soluble properties.
Moreover, in U.S. Pat. No. 4,707,429, the disclosure of which is totally incorporated herein by reference, there are illustrated, for example, liquid developers with an aluminum stearate charge additive. Liquid developers with charge directors are illustrated in U.S. Pat. No. 5,045,425.
Also of relevance with respect to the present invention is U.S. Pat. No. 5,176,980.
In copending patent application U.S. Ser. No. 986,316, the disclosure of which is totally incorporated herein by reference, there is illustrated a process for forming images which comprises (a) generating an electrostatic latent image; (b) contacting the latent image with a developer comprising a colorant and a substantial amount of a vehicle, which developer has a melting point of at least about 25.degree. C., the contact occurring while the developer is maintained at a temperature at or above its melting point, the developer having a viscosity of no more than about 500 centipoise and a resistivity of no less than about 108 ohm-cm at the temperature maintained while the developer is in contact with the latent image; and (c) cooling the developed image to a temperature below its melting point subsequent to development.
In U.S. Statutory Invention Registration No. H1483, U.S. Pat. No. 5,306,591, and U.S. Pat. No. 5,308,731, the disclosures of which are totally incorporated herein by reference, there is illustrated the following: a liquid developer comprised of a certain nonpolar liquid, thermoplastic resin particles, a nonpolar liquid soluble ionic or zwitterionic charge director, and a charge adjuvant comprised of an aluminum hydroxycarboxylic acid, or mixtures thereof; U.S. Pat. No. 5,306,591 discloses a liquid developer comprised of thermoplastic resin particles, a charge director, and a charge adjuvant comprised of an imine bisquinone; and U.S. Pat. No. 5,308,731 discloses a liquid developer comprised of a liquid, thermoplastic resin particles, a nonpolar liquid soluble charge director, and a charge adjuvant comprised of a metal hydroxycarboxylic acid.
Illustrated in U.S. Pat. No. 5,409,796 is a positively charged liquid developer comprised of thermoplastic resin particles, optional pigment, a charge director, and a charge adjuvant comprised of a polymer of an alkene and unsaturated acid derivative; and wherein the acid derivative contains pendant ammonium groups, and wherein the charge adjuvant is associated with or combined with the resin and the optional pigment; and U.S. Pat. No. 5,411,834 is a negatively charged liquid developer comprised of thermoplastic resin particles, optional pigment, a charge director, and an insoluble charge adjuvant comprised of a copolymer of an alkene and an unsaturated acid derivative, and wherein the acid derivative contains pendant fluoroalkyl or pendant fluoroaryl groups, and wherein the charge adjuvant is associated with or combined with said resin and said optional pigment.
Carrier liquids containing commercial mixtures of ISOPARS.RTM. and NORPARS.RTM. (products of Exxon Chemical and of the Sol B series (products of Shell Chemicals) have an initial boiling point of at least 150.degree. C. and boiling point ranges less than 12.degree. C. Such fluids can possess high vapor pressures unless they are scrupulously free of low molecular weight impurities. Isomers of pure hydrocarbons may be suitable, however, such materials are costly. Boiling point and boiling point range does not usually provide a process for the to selection or production of low cost, low viscosity, low vapor concentration carrier liquids, since for example boiling is a macroscopic phenomena concerned with the behavior of the bulk of the material, and vapor concentration can easily be influenced to a large extent by the presence of small amounts of a volatile impurity.
One procedure used to determine the concentration of hydrocarbon mixtures in the gas phase in equilibrium above the liquid is illustrated hereinafter. Normal linear hydrocarbon standards were obtained from Polyscience Corporation, Evanston, Ill.; and HPLC-GC/MS grade methylene chloride was obtained from Fisher Scientific, Rochester, N.Y. A Varian Model 3700 gas chromatograph equipped with a split/splitless capillary column injector and a flame ionization detector was used for the headspace and direct injections. GC separations were prepared using a 60 meter 0.32 millimeter I.D., 1 micron film thickness DB-5 column supplied by J & W Scientific. The GC oven temperature was programmed from 1,000.degree. C. (0 minutes hold time) to 2,450.degree. C. (hold 15 minutes) at a rate of 100.degree. C./minute. The headspace sample injection volume was 2 milliliters. The liquid standard injection volume was 1 .mu.l. A Chromperfect integrator was used for data collection, storage and integration. The samples were heated using a Vanlab block heater supplied by VWR Scientific in Rochester, N.Y. 22 Milliliters screw cap vials with hole caps and septa from Supelco, Inc. (Bellefonte, Pa.) were used to contain and equilibrate the hydrocarbon samples in an aluminum block that was specially made to fit the Vanlab block heater. A 2 milliliter Pressure-Lok (Series A) syringe made by Precision Sampling in Baton Rouge, La. was used for transfer of the headspace gas to the gas chromatograph.
One milliliter of mixed hydrocarbon sample was sealed in the headspace vial and allowed to equilibrate for 30 minutes at the desired temperature prior to analysis. Two (2) milliliters of headspace gas were injected into the gas chromatograph. Peak areas of the detected hydrocarbons were used for quantitation against an external standard curve made using the standards described below.
Ten microliters of each normal linear hydrocarbon were diluted in 10 milliliters of methylene chloride. 1 Microliter of standard was injected into the gas chromatograph. The chromatogram obtained is used to determine the average carbon number for the mixed hydrocarbon gas phase sample and as an external calibration for these gas phase hydrocarbons. The micrograms of hydrocarbon injected into the gas chromatograph are converted to a gas phase concentration of the hydrocarbon using the formula below. The formula was derived from the ideal gas law (PV=nRT) and uses 1 atmosphere (760 millimeters) for pressure, 2,930 degrees Kelvin for temperature and assumes the liquid volume was expanded into 2 milliliters of air (the volume used for the headspace analysis). EQU ppm(v/v)=(density/molecular weight).times.12,019.23
The chromatogram from the headspace gas of the mixed hydrocarbon is compared to the chromatogram from the injection of the liquid standard of normal hydrocarbons. The peak area for the standard normal hydrocarbon nearest the center of the distribution of hydrocarbons in the gas phase is compared to the total peak area of all the hydrocarbons in the gas phase for the purpose of quantitation. This is possible because the distribution of hydrocarbons in the gas phase is usually Gausian and any differences in response of the flame ionization detector due to differing numbers of carbons will average out.
______________________________________ Typical Standard Curve (For a Distribution Centered at Pentadecane) Amount in 10 ml ppm Peak Area ______________________________________ 0 0 0 10 .mu.l 43.48 148380 50 .mu.l 217.40 633530 ______________________________________
If the sum of all the areas for all the peaks in the chromatogram are used, a standard of the normal hydrocarbon where the peaks are centered can be used because the response factor for the F.I.D. should average out over the high and low ends of the distribution (if the distribution is symmetrical).
Using the above process technique the differences in the composition of the liquid phase and the vapor in equilibrium with it can be demonstrated. This point is usefully illustrated by the case of Superla 5NF, a light mineral mineral oil.
This mineral oil material is comprised of an extremely large number of aliphatic hydrocarbons of varied structure including normal chained, branched chain and cyclic materials. The distribution is centered about n-heptadecane and less than 1 percent of the material has a retention time less than that of n-tridecane. The boiling point of this material is greater than 273.degree. C. In spite of this high boiling point, the vapor concentration above this fluid is 50 ppm at 25.degree. C. Examination of the distribution of hydrocarbons found in that vapor are such a small fraction of the composition of the liquid that they are essentially undetectable in that liquid.
Selection of a carrier liquid for use in an electrographic, electrophotographic or other similar printing process, requires detailed knowledge of the device in which the process is to be performed. The variables in the device that must be considered include the type of metering of the carrier liquid and the developed image that will be performed, the speed of the printing process, the temperature at which the printing process is carried out, the temperature of the environment in which the printing device is located, and the mechanical and electrostatic details of that printing process. Generally it can be said, however, that performance closer to optimum will be produced as the viscosity of the fluid is reduced and as the vapor concentration above the fluid at the relevant temperature is decreased, and as the requirement for isomeric purity is relaxed yielding lower cost.
The disclosures of each of the copending applications mentioned herein are totally incorporated herein by reference.